MrKWatkins

A Data Annotations Aware Model Binder

The System.ComponentModel.DataAnnotations assembly added with .NET 3.5 SP1 is quite useful. It contains various attributes that allow you to specify various constraints on the properties of your class. Ideal for model classes; constraints on the data are kept inside your model, nice and succinctly. However ASP.NET MVC version 1 does not contain out of the box server side support for them. Version 2 will but I can't wait that long... Whilst you can use something like the excellent xVal for client side validation of data annotations being a good programmer you'll also want to have the same validation server side in case someone turns off JavaScript in their browser.

Sadly I haven't been able to find a good example of how to do this. The examples I have found tend to require manual calls to some method to perform the validation, such as this one in Steve Sanderson's blog about how to use xVal. I don't want to have to do an extra manual call for my validation; I might forget! The ideal would be a model binder that is aware of the data annotations; I could set it as the default binder and forget about it. The only example of a model binder I could find was in the ASP.NET site on CodePlex but that required a newer version of the DataAnnotations assembly (basically the .NET 4.0 one) which seemed like overkill to me. How hard can it be to write one?

Turns out not very... All you have to do is:

1. Create a class that inherits from DefaultModelBinder.
2. Override the OnPropertyValidating method. Your version should call the base method to get the errors from the default binder then look for any ValidationAttributes on the property, check if there are errors by calling the IsValid method and add them to the ModelState if there are. Something like this:
   

   protected override bool OnPropertyValidating(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor, object value)
   {
   	// Run everything by the default implementation first before checking data annotations.
   	var isValid = base.OnPropertyValidating(controllerContext, bindingContext, propertyDescriptor, value);
   	
   	// Loop through any validation attributes.
   	foreach (var validationAttribute in propertyDescriptor.Attributes.OfType<ValidationAttribute>())
   	{
   		// Are we valid?
   		if (!validationAttribute.IsValid(value))
   		{
   			// No. Let's add the error to the model state and set our isValid status to false.
   			string key = CreateSubPropertyName(bindingContext.ModelName, propertyDescriptor.Name);
   			
   			// Add the error to the model state.
   			bindingContext.ModelState.AddModelError(key, validationAttribute.FormatErrorMessage(propertyDescriptor.DisplayName)));
   			isValid = false;
   		}
   	}
   	return isValid;
   }

3. Replace the default model binder with an instance of your new one via the ModelBinders.Binders.DefaultBinder property.

Okay, it's not quite as simple as that. You also have to take into account any MetadataTypeAttributes that might be found on your model class. This attribute allows you to specify another type to put your metadata on. You can specify identical properties on this other type and decorate them with the DataAnnotations attributes instead. Pointless? Well yes, it should be really; why not just decorate your properties? However it comes in useful if you're working with auto-generated code that doesn't allow you to add attributes, such as the evil and crappy LINQ to SQL designer. By using the MetadataTypeAttribute you can create a partial version of your LINQ to SQL entity class, add the attribute to that and specify constraints for the auto generated properties in some other class. (Whether you should be using LINQ to SQL entities as your model is an ethical debate I am going nowhere near)

If you can't be bothered to do all this of course then please feel free to download one I made earlier. This binder takes into account MetadataTypeAttributes and I've even thrown in a demo project so you can see the thing in action. Enjoy!

Valid XHTML In Redirects

I've been using the Search Engine Optimization Toolkit add-in for IIS 7 to analyse this website. If you've not used it before I recommend you give it a try; it's very good for picking up random bugs, spurious URL routing and dodgy links that you'd never have spotted otherwise. For example it would report issues on this site whereby the same content was accessible by two different URLs. Turns out some links would have a trailing slash, some not. (Mainly due to ASP.NET MVC's ActionLink being rubbish and missing off trailing slashes. Why does it not return links in the format specified by the route???) The SEO toolkit picked them up and now they're hopefully all fixed.

One set of problems I had trouble fixing was the root URL to this site, http://www.mrkwatkins.co.uk/. Currently I only have a blog so that URL redirects to the root URL of my blog, http://www.mrkwatkins.co.uk/Blog/. However the SEO Toolkit would report errors with the content returned from the redirect. For example it would tell me that there was no <h1> tag in the content. Now whether or not the toolkit should be analysing the content returned from a redirect or not I have no idea. What I do know is that they were the last errors on my report and needed removing!

The redirect was done using the standard RedirectResult in ASP.NET MVC. This in turn uses the standard Response.Redirect from ASP.NET. Trouble is that Response.Redirect spits out the following content as well as the 302 status code and location HTTP header:

<html><head><title>Object moved</title></head><body>
<h2>Object moved to <a href="http://www.mrkwatkins.co.uk/Blog/">here</a>.</h2>
</body></html>

So to fix the problems we need to change this XHTML. Turns out that is pretty simple to do. You can return whatever content you like from the request as normal. All you need to do is set the status code to 302 using the Response.StatusCode property and set the Location HTTP header using the Response.RedirectLocation property. I've wrapped all this up in a custom ViewResult class:

using System.Web;
using System.Web.Mvc;
using KWatkins.Validation;

namespace KWatkins.MrKWatkins.Web.Mvc
{
    /// <summary>
    /// A <see cref="ViewResult" /> that redirects the user to another location; allows
    /// you to customize the XHTML returned by the redirection rather than use the
    /// standard (invalid XHTML) retured by <see cref="HttpResponse.Redirect(string)" />.
    /// </summary>
    /// <remarks>
    /// The redirect location is added to the <see cref="ViewRedirectResult.ViewData" />
    /// with the key specified by <see cref="ViewRedirectResult.RedirectLocationViewDataKey" />.
    /// </remarks>
    public sealed class ViewRedirectResult : ViewResult
    {
        /// <summary>
        /// The key used to store the <see cref="RedirectLocation" /> in the
        /// <see cref="ViewRedirectResult.ViewData" />.
        /// </summary>
        public const string RedirectLocationViewDataKey = "RedirectLocation";

        private readonly string redirectLocation;

        /// <summary>
        /// Initializes a new instance of the <see cref="ViewRedirectResult"/> class.
        /// </summary>
        /// <param name="redirectLocation">The redirect location.</param>
        public ViewRedirectResult(string redirectLocation)
        {
            Validate.Argument(redirectLocation, "redirectLocation").IsNotNull().IsNotEmpty();

            this.redirectLocation = redirectLocation;
            ViewData[RedirectLocationViewDataKey] = redirectLocation;
        }

        /// <summary>
        /// When called by the action invoker, renders the view to the response.
        /// </summary>
        /// <param name="context">The context within which the result is executed.</param>
        public override void ExecuteResult(ControllerContext context)
        {
            context.HttpContext.Response.StatusCode = 302;
            context.HttpContext.Response.RedirectLocation = redirectLocation;
            base.ExecuteResult(context);
        }

        /// <summary>
        /// Gets the redirect location.
        /// </summary>
        /// <value>The redirect location.</value>
        public string RedirectLocation
        {
            get
            {
                return redirectLocation;
            }
        }
    }
}

You can then setup a new Redirect method in your base controller to create a ViewRedirectResult for you:

/// <summary>
/// Returns a <see cref="ViewRedirectResult" /> that redirects to the specified URL.
/// </summary>
/// <param name="url">The URL to redirect to.</param>
/// <returns>The <see cref="ViewRedirectResult" /> that redirects to the specified URL.</returns>
protected static new ViewRedirectResult Redirect(string url)
{
    var result = new ViewRedirectResult(url)
                 {
                     ViewName = "Redirect"
                 };
    return result;
}

All that remains is to create the Redirect view. We need this to be valid XHTML with a few extras to keep the SEO Toolkit happy; we need a content type, a description and a <h1>:

<%@ Page Language="C#" Inherits="System.Web.Mvc.ViewPage" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
    <head>
        <title>MrKWatkins - Object Moved</title>
        <meta http-equiv="Content-type" content="text/html;charset=UTF-8" />
        <meta name="description" content="This object has been moved." />
    </head>
    <body>
        <div>
            <h1>Object Moved</h1>
            <p>
                Object moved to <a href="<%= HttpUtility.HtmlAttributeEncode((string)ViewData["RedirectLocation"]) %>"><%= ViewData["RedirectLocation"] %></a>.
            </p>
        </div>
    </body>
</html>

Et voila. The content of your redirects is now valid XHTML and the spurious warnings from your SEO Toolkit report go away.